1. Field of the Invention
The present invention relates to a developing apparatus used for an image forming apparatus, such as a copier or a printer, having a function of forming an image onto a recording material, such as a sheet.
2. Description of the Related Art
An image forming process of an image forming apparatus of a dry one-component electrophotographic system employing a contact developing system will be described with reference to FIG. 19.
Toner (developer) 101 in a developing container 107 of a developing apparatus 102 is carried to a toner carrying member 106, serving as a developer carrying member, by a stirring member 103 and a supplying roller 104. The toner 101 is brought into sliding contact with the contact portion between the toner carrying member 106 and the supplying roller 104 so as to be deposited onto the toner carrying member 106. The deposited toner passes through the contact portion between a regulating member 105 and the toner carrying member 106. In this case, the deposited toner is brought into sliding contact with the contact portion between the regulating member 105 and the toner carrying member 106 so as to be charged and to form a thin layer. Then, the deposited toner passes through the contact portion between the toner carrying member 106 and an image bearing member 113 with the rotation of the toner carrying member 106.
The image bearing member 113 is charged to a polarity which is the same as the polarity of the toner 101 (charging) by a charging roller 114 before the image bearing member 113 reaches the contact portion between the toner carrying member 106 and the image bearing member 113, and exposed by a laser scanner 115 (exposure). Laser 116 in the laser scanner 115 scans the surface of the image bearing member 113 by the rotation of a polygon mirror 117, while controlling light quantity based on the image data. The image bearing member 113 has photoconductivity, so that the potential at the exposed portion is reduced according to the exposure amount. As a result, an electrostatic latent image is formed on the image bearing member 113 due to the potential difference between the exposed portion and non-exposed portion.
The toner carrying member 106 has a polarity which is the same as that of the toner 101, and has applied thereto a voltage having a potential smaller than a potential at the non-exposed portion of the image bearing member 113 and greater than a potential at the maximum exposed portion. When the toner 101 on the toner carrying member 106 is brought into contact with the image bearing member 113, the exposed portion receives force in the direction toward the image bearing member 113 from the toner carrying member 106 by electrostatic force, while the non-exposed portion receives force in the reverse direction. Therefore, when the toner passes through the contact portion, the toner 101 at the exposed portion is deposited onto the image bearing member 113, and the electrostatic latent image is developed as a toner image (developing).
Thereafter, voltage having a polarity reverse to that of the toner 101 is applied to a transfer roller 119 with a recording material nipped between the transfer roller 119 and the image bearing member 113, whereby the toner image on the image bearing member 113 receives electrostatic force directing toward the recording material so as to be transferred onto the recording material (transfer). The recording material having the toner image transferred thereon passes between heated pressure members 121 in a fixing unit 120, thereby being fixed onto the recording material because the toner is melted (fixing). The remaining toner 101 on the image bearing member 113 that is not transferred is removed from the image bearing member 113 by a cleaning member 122, which is in contact with the image bearing member 113, and put into a waste toner container 123 (cleaning).
However, in the image forming apparatus of a dry one-component electrophotographic system, the contact developing system might entail a problem described below when the electrostatic latent image on the image bearing member is developed.
Since the toner carrying member 106 and the image bearing member 113 are brought into contact with each other for developing in the contact developing system, at least one of the toner carrying member 106 and the image bearing member 113 is made of a viscoelastic material in most cases. In many image forming apparatuses, the toner carrying member is made into a viscoelastic material made of a silicon rubber.
The toner carrying member 106 is elastically deformed due to the pressure applied at the contact portion between the toner carrying member 106 and the regulating member 105. When the image forming apparatus is stopped for a long period of time, the regulating member 105 continues to apply pressure to the toner carrying member 106 for a long period of time. In such a case, the portion of the toner carrying member 106 where the regulating member 105 is brought into contact might have permanent deformation, which is not returned to the state before the application of the pressure, due to the compression, even when the toner carrying member is released from the regulating member 105, even when the application of pressure is discontinued, or even when the regulating member 105 is separated.
The permanent deformation due to the compression of the toner carrying member 106 might be generated at the contact portion between the toner carrying member 106 and the image bearing member 113. At this deformed portion, the contact state between the toner carrying member 106 and the regulating member 105 is changed when the deformed portion passes through the contact portion between the toner carrying member 106 and the regulating member 105 through the drive of the toner carrying member 106, so that the amount of the toner per unit area on the toner carrying member might be changed.
In the contact developing system, the amount of toner to be developed is determined by an electric field in the toner layer. Therefore, when the amount of toner on the toner carrying member is changed, the amount of toner to be developed to the same electrostatic latent image is changed. Accordingly, the uniformity of an image density is lowered before or after the deformed portion.
The methods described below have been proposed in order to solve this problem.
(1) Method of reducing a contact force of a regulating member when an image is not formed (Japanese Patent Application Laid-Open No. 2002-333772)
In this method, a driving mechanism is mounted to the regulating member for reducing the contact pressure of the regulating member when the image is not formed. By virtue of this configuration, the deformation of the toner carrying member is reduced, so that the non-uniformity in the image density can be prevented.
(2) Method of applying vibrating electric field to a regulating member (Japanese Patent Application Laid-Open No. 2007-240595)
In this method, storage means provided to the developing apparatus calculates the period when the image forming apparatus is stopped. When the period, when the image forming apparatus is stopped, reaches the period by which the permanent deformation due to the compression is caused, the vibrating electric field is applied to the regulating member. By virtue of this configuration, the toner on the toner carrying member is rearranged through the application of the vibrating electric field to the regulating member, when the toner carrying member passes the regulating member. Thus, the change in the toner amount at the deformed portion on the toner carrying member is reduced, whereby the non-uniformity in the image density can be prevented.
(3) Method of controlling electric current, flowing through a regulating member, to be a constant current (Japanese Patent Application Laid-Open No. 2006-154369)
In this method, voltage is applied to the regulating member having conductivity so as to control electric current, flowing through the regulating member, to be constant. By virtue of this configuration, the current flowing through the regulating member, which depends upon the toner amount on the toner carrying member, is controlled to be constant, whereby the change in the amount of the toner on the toner carrying member is reduced. Thus, the non-uniformity in the image density can be prevented.
However, in the above-mentioned related arts, the problems described below might be generated.
Specifically, in the method, described in (1), of reducing the contact pressure of the regulating member when an image is not formed, the size of the developing apparatus might be increased due to the addition of the driving mechanism for the regulating member.
In the method, described in (2), of applying the vibrating electric field to the regulating member, the regulating member might be vibrated to generate noise, since the vibrating electric field having audible frequency is applied to the regulating member. Further, the toner might fly at the outlet side of the contact portion between the toner carrying member and the regulating member. In this case, the toner unfavorably scatters.
At the deformed portion of the toner carrying member, the regulating member moves with the deformation of the toner carrying member, when it passes the contact portion. Therefore, the electrostatic capacitances of the regulating member and the toner carrying member change, so that induced current also flows through the regulating member. Accordingly, there is concern that there is no correlation between the current flowing through the regulating member and the amount of toner.
In the method, described in (3), of controlling the current flowing through the regulating member to be constant, the current including the induced current caused by the motion of the regulating member is controlled to be constant. Therefore, the change in the toner amount might not be able to be reduced at the deformed portion of the toner carrying member.
As described above, the developing apparatus and the image forming apparatus in the related arts cannot reduce the change in the image density at the deformed portion of the toner carrying member without causing adverse effects, such as the increase in size of the apparatus, noise caused by the vibrating electric field to the regulating member, etc.